I would like my ONU flashed for 3rd Party Router

I have Xfinity 2G fiber and need to be using my own router. Who can I speak to for this to happen? I tried Xfinity support and worked with two people that didn’t know what to do. They could verify what I was telling them but the first guy had a second guy call me back and then he told me it needed to be escalated and that I would get a call back, which hasn’t happened.